In the case of high-pressure discharge lamps, the light is produced when current passes through a gas or metal vapor plasma in an enclosed discharge vessel. In order that the light can be used, for example, for imaging in a projection application, the discharge vessel is arranged in a reflector, which concentrates the light and passes it on to a further optical system.
It is known in this respect that some of the radiation emitted by the discharge lamp is reflected back to the discharge lamp by the following optical system. The electrodes of the discharge lamp partially absorb this reflected-back radiation, whereby additional power is introduced into the electrodes along with the power occurring as a result of the electrical operation. This may have the effect that the electrodes heat up considerably, and the temperatures may become so high as to cause the electrodes to deform. This impairs the functionality of the electrodes, and consequently of the discharge lamp; ultimately, failure of the entire illumination unit may result.
A discharge lamp typically has two electrodes arranged lying opposite each other on the optical axis of the reflector. In order to protect particularly the electrode facing the following optical system from reflected-back radiation, the optical axis of the following optical system is typically tilted by an angle of 10° to 30° with respect to the optical axis of the reflector on which the electrodes are arranged. Nevertheless, an introduction of power caused by reflected-back radiation may still be found to occur.